Machines for implanting semiconductor target wafers with ions have been extensively developed. Such machines typically include an ion source, an ion accelerator structure, an ion beam analyzer that selects an ion species from the source, and a lens for controlling the diameter of the beam. Downstream of the lens is a deflection system for the beam which is traveling in a straight line path along a longitudinal axis. In certain implanters, the deflection system includes a pair of X-Y electrostatic deflection plates, one downstream of the other in the ion beam propagation path. The downstream, usually X, deflection plates deflect the beam from the straight line path through a predetermined angle, typically from five to nine degrees, to remove neutral ions that are not of the desired species from the beam irradiating the target wafer.
The ion accelerating structure typically includes a first accelerator (frequently referred to as a pre-accelerator) between the ion source and the ion beam analyzer and a second accelerator (frequently referred to as a post-accelerator) between the analyzer and the quadrapole lens. The first and second accelerators supply sufficient energy to the ions to implant them to the desired depth in the wafer.
It is desired for certain applications for the amplitude of ion beam currents irradiating the semiconductor wafers to be relatively large, for example, three to four milliamperes. In the past, maximum ion beam currents have been up to typically one milliampere. It has been found, however, that if the same accelerator is used for ion beam currents having high and low levels and high and low energies the high current beam has a tendency to diverge as it propagates with lower energy through the accelerator. There is an appreciable decrease in the current of the ion beam irradiating the target because of this tendency of the beam to diverge. The problem can be solved by decreasing the effective length of the accelerator as the ion beam energy is decreased. It is most desirable for this purpose to maintain a constant voltage gradient in the accelerator for the ion beam, regardless of the beam current magnitude, to prevent spreading of the beam.
In the only prior art device that, to my knowledge considers this a problem, the post-accelerator includes eight ring electrodes that are at different potentials and are equi-spaced from each other along the ion beam path. A high voltage DC source is connected across the electrodes and a high voltage resistive divider has terminals connected to equal valued resistors and to adjacent ones of the electrodes, so there are equal voltage gradients between all of the electrodes. The electrodes connected to the highest voltage terminal of the voltage divider and to the third lowest voltage terminal of the divider are selectively connected together by a shorting bar. When the shorting bar is not connected to the third lowest voltage terminal of the voltage divider, the voltage between the highest and lowest voltage terminals of the divider is variable in steps from 175 kilovolts to 25 kilovolts; the voltage gradient between adjacent electrodes under these circumstances varies from 21.825 kilovolts per electrode to 6.25 kilovolts per electrode. When the shorting bar bridges the electrodes connected to the highest voltage terminal and third lowest terminal of the voltage divider together, the voltage between the highest and the lowest voltage terminals varies in steps from 50 kilovolts to 0 volts; the voltage gradient between adjacent electrodes under these circumstances varies from 16.667 kilovolts per electrode to 0 kilovolts per electrode. The voltage of the pre-accelerator is fixed at 25 kilovolts in this arrangement.
Thus, with this prior art arrangement there are very great differences in the voltage gradients under differing conditions and there is an appreciable step increase in the gradient when the shorting bar initially engages the third lowest voltage terminal of the voltage divider. While this post-accelerator structure was found to function satisfactory for prior art implanters with one milliamp maximum beam current, it is not satisfactory for implanters with maximum beam currents of three to four milliamps because a significant amount of the current from the analyzer diverges as it propagates through the post-accelerator and thereby never reaches the semiconductor wafer being irradiated.
It is, accordingly, an object of the present invention to provide a new and improved charged particle accelerator.
Another object of the invention is to provide a new and improved ion beam accelerator for ion beam implanters.
An additional object of the invention is to provide a new and improved charged particle beam accelerator having a variable effective length and a substantially constant voltage gradient along the length of the charged particle beam.
A further object of the invention is to provide an ion beam implanter with an improved ion beam accelerator capable of accelerating beams of variable amplitude and energies with substantially the same voltage gradient along the length of the beam.